The invention relates to a precipitated silica, a process for its preparation and its use as a matting agent.
It is known that synthetic, precipitated silicas or silica gels can be used as matting agents (DE-PS 24 14 478, DE-PS 17 67 332, DE-OS 16 69 123, DE-AS 15 92 865, DE-A 38 15 670).
The matting power of a silica depends on a variety of factors, such as, for example, the type of silica, the particle size, the particle size distribution, the refractive index and also the lacquer system. The shape and size distribution of secondary particles in the silica are of particular importance.
In addition to being very efficient, expressed by the reduction in degree of gloss as compared with the non-matted lacquer film, a silica which is used as a matting agent also has to satisfy a number of other requirements. Thus, for example, there should be no undue thickening of the lacquer system due to the silica which is introduced. A smooth surface to the lacquer should be produced on the corresponding thin lacquer coatings. Specks which have an adverse effect on the surface quality must be avoided.
The document DE-A 31 44 299 describes precipitated silicas and a process for preparing these precipitated silicas which are characterized by the following physico-chemical properties:
When preparing these silicas, an Alpine transverse flow mill or a jet mill is used to mill the product following spray drying. It is also specified in this document that these precipitated silicas are valuable, highly effective matting agents for lacquers. Precipitated silicas which are prepared using these types of mill lead to disadvantageous roughness of the surface due to the presence of large specks in the final lacquer. The grindometer value (according to ISO 1524) in black stoving enamel is greater than 100 xcexcm and 85 to 90 xcexcm respectively for the known precipitated silicas. Thus these precipitated silicas can only be used to a limited extent as matting agents.
There is now the object of preparing a precipitated silica which does not have these disadvantages.
The object of the invention is a precipitated silica which is characterized by the following physico-chemical parameters:
Size distribution index   I  =                    d        90            -              d        10                    2      ⁢              d        50            
Another subject of the invention is a process for preparing precipitated silicas according to the invention with the physical-chemical parameters:
Size distribution index   I  =                    d        90            -              d        10                    2      ⁢              d        50            
which is characterized in that a precipitated silica which has the following physico-chemical properties
is milled using a classifier mill or a fluidized bed counter-flow mill.
The initial silica is described in the document DE-A 31 44 299.
In one embodiment of the invention, mentioned by way of example, a ZPS classifier mill (Zirkoplex(copyright) Alpine Aktiengesellschaft D-8900 Augsburg) or an AFG fluidized bed counter-flow mill may be used.
In another variant of the invention, the precipitated silica according to the invention may be classified after milling in order to adjust to a specific granular fraction. In a preferred embodiment of the invention, the precipitated silica has the size distribution shown in FIG. 1.
Classifying may be performed, for example, using an ATP Turboplex fine classifier (Alpine Aktiengesellschaft D-8900 Augsburg).
The invention also provides a precipitated silica coated with a polyethylene wax emulsion which is characterized by the following physico-chemical parameters:
This precipitated silica can be prepared by adding polyethylene wax emulsion to a precipitated silica which has the following physico-chemical characteristics:
and then drying and milling the product using a classifier mill or a fluidized bed counter-flow mill.
In one embodiment of the invention, the precipitated silica can be prepared by liquefying the filter cake under the action of shear forces, adding polyethylene wax emulsion, spray drying and then milling using a classifier mill or a fluidized bed counter-flow mill.
A precipitated silica in accordance with DE-A 31 44 299 is preferably used as the starting silica.
Precipitated silica according to the invention has the following advantages:
The advantages of precipitated silicas according to the invention are in particular their high matting efficiency, in addition to further advantages such as the very smooth surface of the dry lacquer, high transparency and a small effect on the rheology (viscosity) of the lacquer.